1. Field of the Invention
This invention relates to an apparatus for detoxifying an exhaust gas by a discharge plasma. More specifically, this invention relates to an apparatus for treating an exhaust gas by which a reactive gas discharged in a thin film forming technique utilizing vacuum, for example, various chemical vapor phase growing methods such as a vacuum chemical vapor deposition (CVD) method, a plasma CVD method, a light CVD method and a plasma etching method is treated with a plasma in vacuum (under reduced pressure).
2. Description of the Prior Art
Reactive gases used in various CVD methods or the plasma etching method for thin film formation utilizing vacuum are not all consumed at the time of film formation, and the unreacted gases remain or by-product gases are generated. Sometimes, in the process of thin film formation, the starting gases may be discharged from a pump without going through CVD or etching. When released into the atmosphere without treatment, many of these reactive gases have a risk of explosion, and some of them are toxic, to cause accidents and air pollution. Accordingly, the permissible concentrations of these gases in the atmosphere are regulated.
Dilution with a large excess of an inert gas, a catalyzed chemical reaction, wet absorption, wet adsorption, and dry adsorption, for example, have been used to detoxify these reactive gases. All of these methods are performed under atmospheric pressure after evacuation by a vacuum pump, and require high costs for equipment and operation. In addition, if proper maintenance is not exercised, safety cannot be secured, and there is a risk of accidents.
As another type of technology, there was proposed an exhaust gas treating method utilizing discharge (discharge treatment method). This method, in contrast to the above-described methods, is characterized by the fact that the gas is treated under vacuum before it is discharged from a pump out of the system. For example, Japanese Laid-Open Patent Publication No. 129868/1976 discloses a treating method by which an exhaust gas containing toxic substances is contacted with an oxidizing agent in a plasma thereby converting the toxic substances into stable compounds, and these compounds are removed from the exhaust gas. Japanese Laid-Open Patent Publication No. 6231/1983 discloses an exhaust gas treating apparatus which is interposed between a reaction vessel from which a reactive exhaust gas is discharged and a discharging device and decomposes the exhaust gas by a discharge and discharges the treated gas. With this type of apparatus, the range of loads capable of maintaining a plasma required for detoxify the exhaust gas to a predetermined concentration is naturally limited, and it is difficult to maintain a stable plasma condition for great load variations, above all pressure variations. The range of application of this apparatus has to be limited.
Recently, there was proposed a method involving utilizing a plasma having a magnetic field superimposed thereon in the discharge treatment method to make it possible to follow load variations (the magnetic field superimposing method) [Society of Applied Physics, Meeting for Studying Plasma Electronics, January 1986]. In the magnetic field superimposing method, the turning radius of an electron in the plasma becomes small, and the electron can turn between electrodes upon application of a dc or ac magnetic field at an angle of about 45.degree. to about 135.degree. to the direction of an electric field generated by the electrodes. Hence, a stable discharge under a load with a broad range of 0.01 m torr to several tens of torr can be maintained.
The discharge treating method, as stated above, is characterized by the fact that the gas is treated under vacuum before it is discharged from the pump out of the system. Inevitably, therefore, the discharge treating apparatus is disposed between the CVD apparatus or etching apparatus and the vaccum discharge pump. Accordingly, the discharge treatment apparatus applicable for practical purposes is required to be as small as possible in size and energy-saving so that it can be easily built between the CVD apparatus or etching apparatus and the vacuum discharge pump. The prior technical literature, however, only discloses the basic concept of superimposing and applying a magnetic field and cause the plasma condition to follow load variations, and does not specifically teach anything on actual conditions for meeting the above requirement, for example, the prescription of optimum structures and sizes of electrodes for generating the plasma. Thus, no exhaust gas treating apparatus which can be used for practical purposes has yet been realized.
It is an object of this invention to provide a discharge treatment apparatus which is sufficiently small-sized and energy-saving for practical application.
The present inventors extensively made investigations in order to achieve this object, and have consequently found that by placing a cathode face to face with a cathode and an anode face to face with an anode with the cathode pair and the anode pair in a specified positional relationship to form a space defined by the cathode pair and the anode pair, preferably providing at least one auxiliary anode approximately parallel to the anode pair in the space defined by the cathode pair and the anode pair, and thereafter applying a magnetic field in the opposing direction of the cathodes, the speed of treating the gas per unit volume of the treating apparatus can be markedly increased and the treating apparatus can be operated stably over long periods of time.